This invention relates to cargo handling and, more particularly, to tow plates for usage in aerial cargo delivery systems. One application of the invention illustrated and described herein pertains to a tow plate for usage in a low altitude parachute extraction system (LAPES).
Low altitude airdrop of equipment or supplies according to the LAPES system is accomplished as a cargo carrying aircraft of the type having a rear-opening cargo compartment flies level at a delivery height of five to ten feet off the ground over a cargo drop location. The cargo is carried on a load bearing platform which rides upon appropriate roller trays mounted by the aircraft floor to provide low-friction load roll out at the delivery point. The loaded platform is pulled rearwardly out of the aircraft cargo compartment by an extraction force exerted by one or more deployed extraction parachutes, descends toward and contacts the ground while decelerating in an essentially level attitude responsive to the extraction force, and slides to a halt, thereby affording highly accurate and reliable cargo delivery. Upon completion of cargo extraction, the aircraft clears the extraction zone and climbs back up to a desired flight altitude.
A typical LAPES system is made up of a drogue parachute, one or more extraction parachutes connected to the cargo, and a force transfer device which is commonly referred to as a "tow plate". The tow plate includes a removable retention assembly mounted on the floor or rear loading ramp of the aircraft, and a clevis assembly connected between the drogue parachute and the extraction parachutes in force transmitting relation therewith. The drogue parachute is deployed as the aircraft descends toward the cargo delivery point and thus exerts a drag force on the clevis assembly. The retention assembly engages the clevis assembly so that this drag force is transferred to the aircraft structure without transmission to the extraction parachutes. As the aircraft begins a gradual flare toward a level flight attitude at delivery height, the retention assembly is operated by the flight crew to release the clevis assembly and, hence, the drag force applied by the drogue parachute is transferred via the clevis assembly to the extraction parachutes. This force transfer must occur before the cargo is to exit the aircraft so that the cargo has sufficient time to move rearwardly toward the rear end of the cargo compartment for delivery at the desired drop location. As will now be appreciated, the tow plate is an important element in the aforementioned LAPES system in that it must be capable of initiating cargo extraction at a precise time in order to ensure accurate cargo delivery. A critical requirement of the LAPES system is that, in the event the airdrop is aborted or the drogue parachute malfunctions, the drogue parachute can be jettisoned before force transfer to the main extraction parachutes occurs. The tow plate thus must provide reliable jettison.
One exemplary tow plate currently in use by the United States Air Force (hereinafter referred to as the "TALC" tow plate) is disclosed in United States Pat. No. 3,801,051. The TALC tow plate is operated by an actuating cable which is connected via a small winch located at the front of the aircraft cargo compartment to a pendant switch. This switch is handed through the cockpit doorway to the co-pilot and is operated by the co-pilot during a LAPES aerial cargo delivery.
The TALC tow plate is generally unsatisfactory for LAPES usage in that differences in cable slack, cable stretch and winch speed adversely affect positive operation of the tow plate at a precise time. Additionally, it is susceptible to fouling and concomitant jamming due to the accumulation of dirt and other debris incidental to loading and unloading of the aircraft. The retention assembly of the TALC tow plate includes opposed friction slides along which the clevis assembly is movable rectilinearly in a direction parallel to the longitudinal axis of the aircraft during release. In many practical applications, these slides collect sufficient debris that the clevis assembly binds or cannot be released in a positive reliable manner. Likewise, the actuating cable is susceptible to fouling and jamming due to debris accumulation and, in the event the tensile force applied by the actuating cable to operate the tow plate becomes of excessive magnitude, the manual controls become inoperative. In many practical LAPES aerial cargo deliveries, therefore, the TALC tow plate yields unsatisfactory cargo extraction.
Additionally, the TALC tow plate lacks versatility in that it is installed as a removable attachment on the upper surface of the aircraft cargo ramp or aircraft floor for LAPES usage and must be removed when the aircraft is converted to another mode of cargo transporation. Similarly, the actuating cable and associated control or drop indicator apparatus must be installed and removed when the aircraft is utilized for LAPES and other cargo transportation modes, respectively. The TALC tow plate thus creates unacceptable time and labor requirements when used in connection with aircraft which are convertible between one mode of cargo transportation and a LAPES mode. It also poses a hazard to the aircraft and flight crew in that the actuator cable and drop indicator apparatus must be strung along the aircraft floor between the cargo roller trays, where they tend to become entangled with rapidly moving load, especially during a LAPES airdrop.
A critical disadvantage of the TALC tow plate is that it tends to provide unreliable jettison of the drogue parachute. To jettison the drogue parachute, the TALC tow plate releases a rear bolt, about which the drogue parachute line is secured, while retaining a front bolt, about which the extraction parachute line is secured, thereby preventing force transfer to the latter. The rear bolt is attached to the clevis assembly adjacent its ends by two movable swing links, each pivoted at one end and securable at the other end by a removable pin. A jettison control mechanism engages and moves these pins outwardly in an axial direction with respect to the ends of the rear bolt, respectively, to release it for jettison purposes. Inasmuch as these pins must be moved under substantial load bearing forces imposed by the drogue parachute, they frequently bind up and sometimes cannot be removed at all, especially in the presence of accumulated dirt or debris, in which case the only way to jettison the drogue parachute is to cut the drogue line. As a consequence, the drogue parachute often cannot be jettisoned rapidly without endangering the aircraft or the flight crew. In the TALC tow plate, moreover, these pins are secured by destructible washers which must be replaced before the tow plate can be reused.
Another generally similar tow plate utilizes a clevis assembly which is removable to effect force transfer by rotative movement with respect to a retention assembly in response to application of a force by the drogue parachute. In this tow plate, the rear end of the clevis assembly includes two spaced apart curved surfaces which bear against mating curved surface formed by the retention assembly. These surfaces are so positioned with respect to the drogue line that the drag force applied by the drogue parachute produces a torque with respect to these curved surfaces which tends to rotate the clevis assembly upwardly and out of engagement with the retention assembly to effect force transfer. An electrically actuated latch assembly applies a counteractive force to the front end of the clevis assembly for selectively retaining it to transmit the drogue parachute force to the aircraft body, without transmission thereof to the main extraction parachutes. To effect force transfer, the latch assembly is disengaged from the front end of the clevis assembly, which thereupon is free to rotate with respect to the aforementioned curved surfaces responsive to application of torque produced by the drogue parachute drag force. This tow plate therefore is not dependent upon sliding friction to obtain release of the clevis assembly to effect force transfer.
The latter tow plate, like the TALC tow plate, tends to provide unreliable jettisoning of the drogue parachute. It includes a main clevis member and a separate drogue clevis member which fits within and is secured to the main clevis member, both including corresponding rear curved surfaces which bear against a common curved surface formed by the retention assembly. The drogue clevis member is secured within the main clevis member by a removable pin located internally of the clevis drogue line securement pin. To release the drogue clevis member, a jettison control mechanism removes this pin to permit rotation of the drogue clevis with respect to the aforementioned curved surface. Like the TALC tow plate, removal of this pin while under load often is accomplished in an unreliable manner, with attendant aircraft and flight crew hazards. Additionally, the latter tow plate, like the TALC tow plate, is mounted as a removable above-floor aircraft attachment.